This invention relates to ultrasonic vibration generation and use.
Conventionally for ultrasonic cleaning an electrical to mechanical transducer, typically a piezo electric device, within a bath of liquid is driven by a fixed frequency oscillatory electrical signal which is used to provide ultrasonic vibrations within the liquid.
A commonly accepted theory explaining ultrasonic cleaning is that the ultrasonic energy creates cavitation bubbles within a liquid where the sound pressure exceeds the liquid vapor pressure at the particular operating temperature and pressure. The theory is that when the cavitation bubbles collapse, which action is very sudden and forceful, peak energy pulses act through the liquid to effect some cleaning result.
In tests now conducted by the present applicant, applicant has found that this mechanism rather than being a primary cleaning mechanism would appear not to be the most important mechanism acting and in fact previous acceptance of this theory has led to an attempt to mainly optimise ultrasonic frequencies so as to attain maximum power output, which causes standing waves to be established with attendant sound "hot spots" which promote cavitation bubbles.
Our tests have shown that if we rather than attempting to optimise power output by frequency selection for a significant period of time to promote cavitation, we arrange input into a cleaning fluid of the ultrasonic vibration in such a way that the energy input is homogeneously distributed throughout the cleaning fluid averaged over a short period of time then a very significantly improved cleaning effect can be achieved without having to increase the energy input required from the electronic power supply.
Further however this allows for a substantial reconsideration of the power supply necessary because of the reduced power requirements.
In U.S. Pat. No. 4,736,130 Puskas discloses an apparatus with seven controllable variables. These are
1.) the time duration of a power pulse train, which is followed by a
2.) time period of no activity for degassing,
3.) the time duration of individual power bursts during the power train period,
4.) the time duration of periods of no activity between the individual power bursts,
5.) the range of amplitude modulation of each power burst,
6.) the mean transmitted frequency, and
7.) a frequency modulation index.
Puskas states that in regard to 7.) "minimum and maximum frequencies of the sweep frequency function are preferably within a resonant range of the transducer." No limits are imposed on the frequency sweep rate.
In U.S. Pat. No. 4,398,925 Trinh et al. discloses an ultrasonic transmitting apparatus for removing bubbles in a fluid. It is disclosed that the transmitted frequency is swept from 0.5 kHz to 40 kHz and that the ratio between the low and high frequency limit should be at least 10 times. The sweep rate is "slow enough so that each bubble oscillates at least several cycles." U.S. Pat. No. 4,398,925 further teaches that if each frequency sweep is constrained to take about 10 seconds or more, then after about 15 minutes of continuous sweeping, most bubbles will be removed.
In U.S. Pat. Nos. 3,648,188, and 4,588,917 Ratcliff discloses a power oscillator with different resonant arrangements and positive feedback components to cause oscillation.
U.S. Pat. No. 4,864,547 describes means of producing a soft start and means to vary the power to the transducer.
Several phase locked loop arrangements are described so that a resonant frequency of the transducer is locked onto by the drive electronics. U.S. Pat. No. 4,748,365 is an example of this which describes means for searching for the load resonance point and then locking onto it.